• journal_title：The Canadian Mineralogist
• Contributor：Carol D. Frost ; B. Ronald Frost ; Donald H. Lindsley ; Kevin R. Chamberlain ; Susan M. Swapp ; James S. Scoates
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.4.925
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：4
• firstpage：925
• section：Articles

The Laramie anorthosite complex includes three anorthositic plutons: the Poe Mountain, Chugwater, and Snow Creek intrusions. The Poe Mountain and Chugwater bodies exhibit mappable magmatic stratigraphy that records fractionation processes in a magma chamber at or near the present level of exposure followed by doming and expulsion of residual liquid. The Snow Creek pluton is more poorly exposed, although layering is locally observed, and there is geochemical evidence for removal of interstitial liquid in the eastern portion of the body. The three plutons were emplaced in close succession. The U–Pb zircon and baddeleyite geochronology of anorthosite from the Chugwater and Snow Creek plutons, and a monzodiorite dike, indicates that the Chugwater is the oldest of the bodies, with a weighted average ²⁰⁷Pb/²⁰⁶Pb age of five samples of 1435.5 ± 0.3 Ma. The Snow Creek anorthosite intrudes the previously dated 1434 Ma Poe Mountain anorthosite and is, in turn, cut by the 1432.8 ± 2.4 Ma monzodiorite dike. The LAC and associated granites of the Sherman batholith suite were emplaced over a period no greater than 12 million years, and possibly in as little as three million years. Despite their similar ages, each of the LAC anorthosite plutons displays a distinctive assemblage of minerals. The Poe Mountain anorthosite is characterized by olivine, augite, low Ca-pyroxene, ilmenite and magnetite, and plagioclase in the range An_43–53. The Chugwater anorthosite generally contains no olivine but is composed of augite, low Ca-pyroxene, ilmenite and magnetite, and iridescent plagioclase in the range An_50–56. The Snow Creek anorthosite is characterized by a lack of olivine and magnetite and the presence of iridescent plagioclase of composition An₄₇ to An₅₆; it is commonly quartz-bearing. These assemblages record a range of silica activities and oxygen fugacities. Higher activities of silica and fugacities of oxygen are correlated with greater amounts of crustal assimilation, as indicated by Nd and Sr isotopic compositions and the presence of inherited components in zircon. We demonstrate that crustal assimilation can produce a range of mineral assemblages and Sr–Nd isotopic compositions in LAC anorthosites crystallized from common mantle-derived parental magmas, and that assimilation of crust is likely an important control on the compositional variations documented in other Proterozoic massif anorthosites.